Electromagnetic actuator with rotary core

ABSTRACT

Electromagnetic actuator with rotary core, comprising, housed in a casing: a magnetic yoke, a pair of fixed adjacent coils alternately energized, and a rotating element with a magnetic core; the yoke, the coils and the core having a common axis coinciding with that of the actuator, and the core being positioned between and/or within said coils, so as to close alternative circuits of magnetic flux, generated in the yoke by either one of the coils, between opposite axial pole pieces and central radial pole pieces of the yoke. In the actuator: the yoke (2) comprises a single radial pole piece (11) common to both coils (3, 4) and two sets of axial end pole pieces (13, 14). The core (7) comprises a continuous central part magnetically cooperating with the radial piece (11) of the yoke (2) and the two opposite sets of pole pieces (15, 16) axially projecting from the central part and magnetically cooperating with the sets of axial pole pieces (13, 14) of the yoke (2); in the two sets of pole pieces (15, 16) of the core (7), or alternatively in the two sets (13, 14) of the yoke (2), the pole pieces of one set are offset in respect of the pole pieces of the other set.

The present invention concerns an electromagnetic actuator with rotarycore, the rotation of which is controlled directly by the magneticfield.

As known, it is always more widespread in the technique applied to theconstruction of plants and machines to use actuators, namelydevices--electromagnetic, pneumatic, oledionamic--adapted to producesmall linear or angular movements of component parts of said plants andmachines.

The fundamental characteristic of an actuator is the capability toimpart, simultaneously with the motion of a movable element thereof, athrusting and/or drawing force in the case of the linear type ofactuator, or a torque in the case of an angular type of actuator.

The most widespread use of actuators is made of in servomechanisms sincethere is in fact a great advantage--compared to other operatingsystems--in being able to produce the motion where it is directlyrequired, without having to derive it with kinematic mechanisms from acentral motion source.

Among the known actuators, use is particularly made of the electricallyoperated ones, usually defined as electromagnetic actuators or, inshort, as electromagnets.

Obviously, also in the field of electromagnetic actuators there are twotypes of movements, linear or angular, adapted to be imparted therewith.

In the case of a linear movement, the movable element of theelectromagnet (or movable core) performs a motion along a rectilineartrajectory, under the action of a magnetic field obtained by suitablyenergizing one or more coils.

In the case of an angular movement, the movable core of theelectromagnet performs a rotary motion about its own axis, under theaction of a suitable magnetic field.

The technique to produce electromagnets with a linear movement of themovable element has greatly evolved during the past years, and a greatnumber of modifications have been introduced to improve the product soas to be able to obtain more and more advanced performances.

Definitely less evolved is the technique to produce electromagnetswherein the movable core performs a rotary motion about its on axis, or"electromagnets with rotary core". In fact, the examples of constructionof electromagnets with rotary core are positively few, since it isusually preferred--as far as can be reckoned from the proposals of theconstructors--to resort to electromagnets, wherein the rectilinearmovement of the movable core is turned into a rotary motion bymechanical means.

A clear example is represented by the solution proposed by variousconstructors according to which, even though the coils generate a forcealong the axis of the electromagnet, which thus tends to move themovable core along this trajectory said core then performs a rotarymotion thanks to a torque imparted thereto as a result of its motion.

In fact, in said actuators, a movable cylindrical core carries at oneend a disc flange fixed thereto, which faces a circular flat zone of thefixed par of the electromagnet. Between the disc of the movable core andthe circular zone of the fixed par there are interposed some ballsengaging into grooves which extend into a circle arc, said grooves beingformed on the two facing zones in such a way that, when theelectromagnet is energized, the magnetic force induced in the movablecore in an axial sense generates, at the points of contact with theballs, a component perpendicular to the radius and creates a torqueacting on the movable core.

In this way, the movable core performs a rotary motion simultaneouslywith its axial movement.

As can be easily understood, this is a very rough solution whichprejudices the speed of the electromagnet, due to the considerableinertias of the movable core and the significant frictions beingproduced between the various parts.

Said solution is therefore not adapted to solve the great number ofproblems arising in present technique, especially when the times ofresponse of the actuators have to be very short and said actuators haveto perform a large number of consecutive operations.

The present invention now proposes to solve these problems and tosatisfy the present requirements in the field of rotatingelectromagnetic actuators, by supplying an actuator of this type whereinthe rotary core is caused to rotate directly by the magnetic fieldgenerated by the coils of the actuator, without the interposition of anytype of mechanisms, with great advantages as far as simplicity ofconstruction, reliability and the overall performances of the actuatoritself.

It is of course already known to cause electromagnetically the rotationof a core about its axis: this in fact takes place in all the electricrotary motors, which are besides foreseen to impart to the core and itsshaft a long rotation of a high number of revolutions (often producinghigh or very high power), and not the short and precise movements meantto be performed by the actuators (with minimum power requirements).

Among the electric rotary motors having characteristics closest to thoseof the actuators, mention should be made of the "step-by-step" electricmotors, which are adapted to perform rotations of limited or verylimited amplitude, in both directions, often even repeated severaltimes.

An electric rotary motor of this type, defined as electric impulsemotor, is known from U.S. Pat. No. 2,343,325. This is most likely one ofthe first examples of "step-by-step" electric motors. Said motorcomprises--housed in a casing--a magnetic yoke, a pair of fixed adjacentcoils alternately energized, and a core rotating on an axis common tothe coils and being positioned between them, so as to close alternativemagnetic circuits--generated by the energized coil--between oppositeaxial pole pieces and central radial pole pieces of said yoke.

Said motor structure is however by no means suited to form anelectromagnetic actuator adapted to satisfy the present requirements,both from the technical and from the commercial point of view. As wellas being scarcely compact and quite complicated as far as construction,the motor of U.S. Pat. No. 2,343,325 like any other "step-by-step"motor:

is not adapted to simultaneously impart a high torque and a highacceleration, as is indispensable in actuators ("step-by-step" motorssupply in fact a limited torque when allowing high accelerations, andviceversa);

to be used as an actuator it would require costly and complicatedelectronic pilot systems, especially if high accelerations are needed(whereas, actuators have to be as simple as possible, reliable andeconomic);

it involves the risk of an easy loss of synchronism (especially ifcaused to operate with high torques and accelerations), this drawbackbeing incompatible with the functions of an actuator.

All these drawbacks are instead fully overcome in the actuator of thepresent invention, which provides furthermore all the advantages ofactuators with rotary core.

Said actuator--which essentially comprises, housed in a casing, amagnetic yoke, a pair of fixed adjacent coils alternately energized, anda rotating element with magnetic core, the yoke, the coils and the corehaving a common axis coinciding with that of the actuator, and the corebeing positioned between and/or within said coils, so as to closealternative circuits of magnetic flux, generated in the yoke by eitherone of said coils, between opposite axial pole pieces and central radialpole pieces of said yoke--is characterized in that: the yoke comprises asingle radial pole piece, common to both coils, and two sets of axialend pole pieces; the core comprises a continuous central part,magnetically cooperating with the radial pole piece of the yoke, and twoopposite sets of pole pieces axially projecting from said central partand magnetically cooperating with said sets of axial pole pieces of theyoke; in the two sets of pole pieces of said core, or alternatively ofsaid yoke, the pole pieces of one set are offset in respect of those ofthe other set.

In this actuator, said sets of pole pieces of the yoke and of the corecomprise an equal number of uniformly distributed pole pieces, meansbeing provided to prevent a perfect contraposition of the pole pieces ofthe yoke with the pole pieces of the core.

Furthermore, the magnetic gap between the single central pole piece ofthe yoke and the core of the actuator preferably extends along acylindrical surface, while the magnetic gaps between the end pole piecesof the yoke and those of the core magnetically cooperating therewithextend along opposed conical surfaces, said cylindrical surface and saidconical surfaces having their axis coinciding with that of the actuator.

Means can also be suitably provided to limit the angular rotation of thecore.

The invention will now be described in further detail, by mere way ofexample, with reference to a preferred embodiment thereof, illustratedon the accompanying drawings, in which:

FIG. 1 is a diagrammatic axial section view of the electromagneticactuator according to invention; and

FIGS. 2 and 3 are diagrams illustrating the working principle of theactuator shown in FIG. 1.

As shown on the drawings, the electromagnetic actuator of the presentinvention comprises a casing 1, which incorporates a magnetic yoke 2 andhouses two fixed adjacent coils 3, 4, and a movable element 5 formed bya shaft 6 of non-magnetic material and by a magnetic core 7. The shaft6, projecting with an end 6A from the casing 1, is mounted therein ontwo roller bearings 8, 9, so that the movable element 5 is free torotate about an axis 10 common to said shaft, to the yoke 2 and to thecoils 3, 4 (axis of the actuator).

The magnetic yoke 2 surrounds the coils 3, 4, and comprises, a singlecentral radial pole piece 11, common to both coils and magneticallycooperating with the central continuous part of the core 7 by way of amagnetic gap 12 of constant width, which extends along a cylindricalsurface whose axis coincides with the axis 10 of the actuator; and twosets of axial end pole pieces 13, 14 flux linked only with the coil 3and with the coil 4 respectively, and facing each other (see especiallyFIGS. 2 and 3) along the axis 10. An equal number of uniformlydistributed axial pole pieces 13, 14, is provided in the two sets.

Also the magnetic core 7 of the movable element 5 comprises two sets ofpole pieces 15, 16, which axially project from its continuous centralpart and are designed to magnetically cooperate with said sets of facingpole pieces 13, 14, of the yoke 2. Also in this case, an equal number ofuniformly distributed pole pieces 15, 16, is provided in the two sets.Furthermore, their number is equal to that of the sets of pole pieces13, 14, of the yoke 2.

In the embodiment shown, the yoke 2 and the core 7 of the movableelement 5 are configured so that their pole pieces are substantially incontraposition--as clearly shown in FIG. 1--forming magnetic gaps 17,18, which extend along opposed conical surfaces with axes coincidingwith the axis 10 of the actuator.

According to the invention, either in the yoke 2 or in the core 7 thepole pieces of one set are offset in respect of those of the other set.For example, in the embodiment shown, the pole pieces 15 of the core 7are offset--see FIGS. 2 and 3--in respect of the pole pieces 16, whilein the yoke 2 the pole pieces 14 are almost exactly opposite to the polepieces 13.

To operate the actuator of the invention, the coils 3 and 4 arealternately energized and the magnetic flux, generated by only one ofsaid coils at a time, passes through alternative magnetic circuits whichare closed by the rotary core 7 between the central radial pole piece 11and, alternately, the axial pole pieces 13, 14, of the yoke 2.

With an actuator like the one described heretofore, in restconditions--according to FIG. 2--the energizing of the coil 3 producesthe magnetic flux F1 (FIG. 2). For the magnetic circuit crossed by saidflux to take up the configuration of less resistance, the pole pieces 15of the core 7 should move exactly in correspondence of the pole pieces13 of the yoke 2: this is actually obtained by a rotation of the movableelement 5, which is just the effect meant to be produced.

From the position thus reached--shown in FIG. 3, with reference to thecore 7--the movable element can be caused to rotate in a directionopposite to the previous one and can be moved back to its initial restcondition by energizing the coil 4. In fact, in so doing, one generatesa flux F2 (FIG. 3) which linking with the pole pieces 14 of the yoke 2and 16 of the core 7, which are now offset, moves back said pole piecesone in front of the other by rotating the element 5 in a directionopposite to the previous one.

Structural expedients--such as a suitable, slight, reciprocaldisplacement of the pole pieces which are not offset (13 and 14)--favourthe rotation only in one sense, while the angular rotation can belimited by adopting mechanical stops (not shown). Moreover, in theactuator according to the invention, the number of facing pole pieces ofthe yoke and of the core determines the angular step and, consequently,the width of the rotation angle of the movable element.

The actuator of the present invention can be used in a wide number andrange of industrial applications; it is however particularly suited inall those cases in which the actuator is required to perform very fastoperations, with limited movements, and must also be of reduceddimensions. A typical example of this type of application--not meanthowever to limit the scope of the present invention--is the control ofdevices to recover the tension of the weft yarn being fed to looms.

It is understood that the aforedescribed embodiment of the actuatoraccording to the invention is merely an example and in no way limits thepossibilities to realize other embodiments thereof by introducingmodifications and variants: in particular, use could be made of twocoils of different characteristics, in the event that the rotation ofthe movable element in one sense should require more power than itsrotation in the opposite sense. Also more than one pair of coils couldbe provided, and one could vary the shape and mechanical characteristicsof the movable element, as well as its mounting, or the shape andcharacteristics of the sets of cooperating pole pieces,and--obviously--their number.

We claim:
 1. In an electromagnetic actuator with rotary core,comprising, housed in a casing: a magnetic yoke (2) having a centralradial pole piece (11) and two sets of opposite axial end pole pieces(13, 14), a pair of fixed adjacent coils (3, 4) alternately energized,and a rotating element with a magnetic core (7); the yoke (2), the coils(3, 4) and the core (7) having a common axis, the central radial polepiece of the yoke (2) consisting of a single pole piece (11) common toboth coils (3, 4) and the core (7) being adapted to close alternativecircuits (F1, F2) of magnetic flux generated in the yoke (2) by eitherone of said coils (3, 4) between said opposite axial end pole pieces(13, 14) and said central radial pole piece (11) of the yoke (2); theimprovement wherein the core (7) comprises a continuous central partmagnetically cooperating with the central radial pole piece (11) of theyoke (2) and the opposite sets of pole pieces (15, 16) axiallyprojecting from said continuous central part and magneticallycooperating with said sets of opposite axial end pole pieces (13, 14) ofthe yoke (2); and in the two sets of pole pieces (15, 16) of said core(7), or alternatively in the two sets (13, 14) of said yoke (2), thepole pieces of one set are offset in respect of the pole pieces of theother set.
 2. An electromagnetic actuator as claimed in claim 1, whereinsaid sets of pole pieces (13, 14) of the yoke (2) and said sets of polepieces (15, 16) of the core (7) comprise an equal number of uniformlydistributed pole pieces, means being provided to prevent a perfectcontraposition of the pole pieces of the yoke with the pole pieces ofthe core.
 3. An electromagnetic actuator as claimed in claim 1, whereinthe magnetic gap (12) between the single central pole piece (11) of theyoke (2) and the core (7) extends along a cylindrical surface, while themagnetic gaps (17, 18) between the end pole pieces (13, 14) of the yokeand those (15, 16) of the core magnetically cooperating therewith extendalong opposed conical surfaces, said cylindrical surface and saidconical surfaces having their axis coinciding with the axis (10) of theactuator.
 4. An electromagnetic actuator as claimed in claim 1, whereinmeans are provided to limit the angular rotation of the core (7).
 5. Anelectromagnetic actuator as claimed in claim 1, each said set ofopposite axial and pole pieces comprising a plurality of pole pieces.